Proportional-integral-derivative (PID) control is a widely used control method in plant control apparatuses that control plants such as a power plant or a chemical plant. There are two kinds of PID controls: position-type PID control and velocity-type PID control.
Of the above-mentioned two control methods, the velocity-type PID control method that locates an integral calculation unit on the lowermost stream side has the advantage of dispensing with tracking processing.
However, the velocity-type PID control has the following problem. For example, when a plurality of operation terminals arranged in parallel in the piping configuration of a plant are simultaneously operated by one velocity-type PID calculation unit, the difference of values initially made between operation amount commands for the respective operation terminals remains as it is, and has an influence on the subsequent control of the plant. This will be explained with reference to FIG. 7 and FIG. 8.
FIG. 7 is a diagram showing an example of a conventional plant control apparatus. The plant control apparatus shown in FIG. 7 is configured so that two operation terminals 2A and 2B arranged in parallel in the piping configuration of a plant PT are simultaneously operated by one velocity-type PID calculation unit 5. As primary components, the plant control apparatus includes a first deviation calculation unit 4, the velocity-type PID calculation unit 5, an A-integral calculation unit 6A, a B-integral calculation unit 6B, an MV-A overwrite unit 7A, and an MV-B overwrite unit 7B. In the example shown in FIG. 7, both the operation terminals 2A and 2B to be operated are control valves having an operating range of 0% to 100%.
The first deviation calculation unit 4 calculates a deviation between a set value SV which is a desired value of control and a process value PV which is a control target, and outputs a deviation (e). The process value PV is sent from a sensor 3 of the plant PT. The velocity-type PID calculation unit 5 performs a velocity-type PID calculation on receipt of the deviation (e) from the first deviation calculation unit 4, and outputs a velocity-type operation amount command signal SΔMV corresponding to the deviation (e). Both the A-integral calculation unit 6A and the B-integral calculation unit 6B are supplied with the operation amount command signal SΔMV, and output an A-operation terminal position command signal S(MV-A) and a B-operation terminal position command signal S(MV-B) represented by 0% to 100% in accordance with an integral calculation. In this way, the integral calculation is processed by the operation amount command signal SΔMV from the velocity-type PID calculation unit 5, and the A-operation terminal position command signal S(MV-A) or the B-operation terminal position command signal S(MV-B) is thereby generated. This condition is referred to as an automatic mode.
When, for example, an operator has performed manual setting, the MV-A overwrite unit 7A and the MV-B overwrite unit 7B send the manually set values to the A-integral calculation unit 6A and the B-integral calculation unit 6B, respectively. These calculation units perform overwrite processing to transform the A-operation terminal position command signal S(MV-A) and the B-operation terminal position command signal S(MV-B) into signals of the manually set values. In this way, the A-operation terminal position command signal S(MV-A) or the B-operation terminal position command signal S(MV-B) are determined by the operator's manual setting. This condition is referred to as an overwrite mode.
In such a conventional plant control apparatus that uses the velocity-type PID calculation unit, for example, the operator performs manual setting from the MV-A overwrite unit 7A. In this case, the A-integral calculation unit 6A enters the overwrite mode to perform overwrite processing using the manually set value. On the other hand, the B-integral calculation unit 6B continues the automatic mode to perform the integral calculation of ΔMV from the velocity-type PID calculation unit 5. Consequently, the signal S(MV-A) and the signal S(MV-B) become to represent position command values that are different to each other.
If the manual setting by the MV-A overwrite unit 7A is cancelled from this condition, the A-integral calculation unit 6A is switched from the previous overwrite processing with the use of the manually set value to the integral calculation processing of the operation amount command signal SΔMV fed from the velocity-type PID calculation unit 5. However, in this case, there is a difference of the initial values of the integral processing between the A-integral calculation unit 6A and the B-integral calculation unit 6B. Therefore, as shown in the example of FIG. 8, the difference of the initial values remains as it is between the A-operation terminal position command signal S(MV-A) and the B-operation terminal position command signal S(MV-B) which are subsequent calculation results. Accordingly, if the operation amount command signal SΔMV keeps the same positive value in the example of FIG. 8, the operation terminal 2B to which the B-operation terminal position command signal S(MV-B) is input is fully opened earlier than the operation terminal 2A to which the A-operation terminal position command signal S(MV-A) is input. Thereafter, the A-operation terminal position command signal S(MV-A) alone is sent. However, this situation is substantially the same as a situation in which, at the time when the operation terminal 2B has reached its full opening, switching is made from the existing control by two valves to the control by one valve. This leads to the problem of the influence on the control of the plant. It is to be noted that the reason why FIG. 8 shows a condition in which the operation amount command signal SΔMV keeps a positive constant value is to make the above explanation clearer.